As compact products, such as mobile communication terminals and electronic pocket notes become more popular, the demand for a micro integrated non-volatile recording medium increases. It is not easy to downsize existing hard disks nor to highly integrate flash memories. Therefore, an information-storage device using a scanning probe has been studied as an alternative.
Probes are used in various scanning probe microscopy (SPM) techniques. For example, probes are used for a scanning tunneling microscope (STM) which detects current produced when voltage is applied between a probe and a sample to reproduce information, an atomic force microscope (AFM) which uses an atomic force between a probe and a sample, a magnetic force microscope (MFM) which uses an interaction force between a magnetic field from a sample and a magnetized probe, a scanning near-field optical microscope (SNOM) which overcomes a resolution limitation due to the wavelength of visible light, and an electrostatic force microscope (EFM) which uses an electrostatic force between a sample and a probe.
In order to record and reproduce information at high speed and density using such SPM techniques, a probe should be capable of detecting surface charge existing in a region of several tens of nanometers in diameter and being fabricated into an array of cantilevers to improve the speed of recording and reproduction.
FIG. 1A is a perspective view of a probe of a scanning probe microscope with a metal-oxide semiconductor field-effect transistor (MOSFET) channel structure as disclosed in Korean Laid-open Patent Publication No. 2001-45981. FIG. 1B is an enlarged view of a portion “A” in FIG. 1A.
Referring to FIG. 1A, a probe 10 formed by etching a semiconductor substrate 20 protrudes in the form of a cantilever from the semiconductor substrate 20. The probe 10 is connected to electrode pads 20a and 20b located at either side of the probe 10.
Referring to FIG. 1B, a source region 11 and a drain region 13 are formed at inclined surfaces of a V-shaped tip of the probe 10 and a channel region 12 is formed in the middle of the tip between the source region 11 and the drain region 13.
Since the tip of the probe 10 is located at the end of the cantilever, it is difficult to fabricate the probe with tip having a radius of several tens of nanometers into an array form. In the prior art, probes with a tip having a radius of several tens of nanometers have been fabricated through several processes including an oxidation process so that the tip is vertically formed on a cantilever.
However, after a tip of several micrometers in height has been formed, the precision of a subsequent photolithographic process considerably deteriorates. As a result, it is difficult to form a source region and a drain region with a short channel interposed therebetween. Further, even though a short channel can be realized through a diffusion process, it is also difficult to align the short channel at the end of the center of the tip due to alignment errors caused during the photolithographic process.
FIG. 2A and FIG. 2B are schematic cross-sectional views for explaining a method of reproducing information using a MOSFET tip in which a source electrode 11 and a drain electrode 13 are formed.
Referring to FIG. 2A, the V-shaped MOSFET tip 10 doped with p-type impurities includes the source region 11 and the drain region 13 doped with n-type impurities formed at inclined surfaces thereof. The conventional MOSFET tip 10 is moved over the surface of a recording medium 15 to detect current flowing along a channel 12 in response to the polarity of surface charge 17. In this way, the polarity and intensity of the surface charge 17 are determined.
FIG. 2B is an enlarged view of the peak of the MOSFET tip 10, schematically illustrating a state in which a depletion region 14 is diffused.
Referring to FIG. 2B, when the tip 10 of the probe is positioned above a positive surface charge 17 inside the recording medium 15, holes of the channel region 12 doped with p-type impurities are depleted at the end of the tip due to the electric field of the positive surface charge 17.
When the electric field is stronger than an electric field which causes the depletion region to reach its maximum size, a channel of electrons which are minority carriers is formed at the end of the tip. When an even higher electric field is applied, a channel of electrons contacting the source region 11 and the drain region 13 is formed, and current flows through the channel due to voltage applied between the source region 11 and the drain region 13.
That is to say, the conventional MOSFET tip functions as a transistor only when the electric field generated by the surface charge is higher than a threshold electric field value which can form a channel of minority carriers up to the source region and the drain region. Since surface charge generating an electric field smaller than the threshold electric field value cannot be detected, the MOSFET tip operates within a limited range and the sensitivity of the MOSFET probe 10 is low.